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#RosettaWatch: Philae lander reveals comet 67P’s fluffy surface

By Jacob Aron

A good place to make nail-polish remover (Image&colon; ESA)

Everyone’s favourite comet lander has just turned in its homework. Team members have been releasing images and other data from the European Space Agency probe ever since its dramatic landing on comet 67P/Churyumov–Gerasimenko last November, but it wasn’t until today that their first detailed findings were available – spread across eight papers in the journal Science.

Instead of making a soft landing as planned, Philae bounced across the surface of the comet, meaning the team had to reconstruct its trajectory in order to make sense of the instrument readings. One paper dissects the protracted landing&colon; an initial bouncing touchdown, followed by a collision with a crater rim and two more touchdowns.

“This is the official record of what happened when,” says Jens Biele, Philae payload manager at the German Aerospace Center in Cologne. “These bounces, although not intended, could be used to learn something about the mechanical properties of the comet’s surface material.”

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The analysis confirms initial assessments that Philae came to rest in an area of rigid ice, having made its first touchdown in a much softer spot – one probably coated in a layer of granular material at least 20 centimetres deep.

Comet-sniffing

Pictures of that initial landing spot, taken by Philae’s orbiting companion Rosetta, suggest that the lander’s feet created three craters, each a few metres wide. Due to the comet’s low gravity, the impact spread material out almost as if it was liquid, displacing nearly twice the probe’s 100-kilogram mass in dust.

The unorthodox landing meant some instruments weren’t able to operate as planned, in particular the COSAC and Ptolemy spectrometers, designed to analyse samples of cometary material. The lander’s failure to drill into the surface meant they had to fall back into “sniffing” mode, studying whatever material happened to drift into the instruments.

COSAC detected 16 types of molecules on the surface, including trace amounts of four that have never been seen on comets before. “Seeing them from afar is a bit difficult,” says COSAC lead Fred Goesmann. “We had the nice advantage of being there.”

All 16 can be explained by reactions in which water, carbon monoxide, methane and ammonia combine to form more complex molecules. But the four previously unseen compounds – methyl isocyanate, acetone (best known as the chief ingredient in nail polish remover), propionaldehyde and acetamide – were not unexpected, says Goesmann. “I would have hoped for more complicated molecules,” he says.

The team saw no signs of amino acids, the building blocks of proteins. But they are probably there, since they showed up in samples from NASA’s Stardust mission, which returned material from the tail of comet Wild 2 to Earth.

Streaming tail

The cameras on Philae fared slightly better. The downward-pointing ROLIS captured pictures of a tail-like feature streaming behind a large boulder on 67P called Cheops as Philae descended. These streams are thought to be produced by splashing particles rather than any wind. The team is still analysing unreleased close-up pictures taken as Philae sat on the surface.

Philae’s other imaging system, a panoramic camera called CIVA, provided the pictures of Philae’s legs on the surface of 67P that wowed the world last November. The images have also led to important discoveries, says CIVA lead Jean-Pierre Bibring.

Prior to Rosetta and Philae, researchers thought comets were dirty snowballs – mostly ice cementing together darker material. But CIVA shows that the comet’s surface seems to be dominated by grains of fluffy material composed of the kinds of molecules detected by COSAC and Ptolemy, not ice. These grains probably originated in the cloud of material that went on to form our solar system.

There are a few bright grains in the CIVA pictures that still need explanation – they could be ice, or some other material that just happens to be catching the light. If Philae revives and transmits more images, the differing light conditions should provide answers. Bibring is also waiting to use optical and infrared microscopes on Philae that didn’t have enough power to operate in November.

Deferred analysis

The chance of reviving Philae and taking more readings from 67P means that many of the science teams are holding off fully analysing their data until they have a complete picture. Since making contact with Philae in June, mission managers have struggled to maintain a radio link between the lander and Rosetta, possibly because one of its transmitters is damaged.

Now, as the comet nears its closest approach to the sun, Rosetta is studying its southern hemisphere and has no hope of hearing from Philae for a few weeks. Outgassing from the warming comet also means Rosetta has to keep hundreds of kilometres from its surface, whereas the radio link was designed to have a range of 30 kilometres.

Nevertheless, the team remains hopeful that they will able to get Philae up and running again. “If the lander gets the signal and has enough power it will answer and try to establish a link,” says Biele. “We might have a surprise again&colon; the lander always surprises us.”